Liquid control jet during part load operation in a hydraulic turbine

ABSTRACT

A hydraulic turbine including: a passageway permitting liquid to pass through the turbine; a draft tube defining a portion of the passageway through which liquid normally flows in a vortex flow path during optimal turbine operating conditions; a rotatable runner mounted upstream of the draft tube and rotating about a central axis passing through the runner and extending into the draft tube; at least one nozzle head device positioned relative to the central axis of the runner and adjacent to an upper portion of the draft tube, the at least one nozzle head device has at least one nozzle from which a corresponding control jet of high velocity liquid is injected axially downstream of the runner and into liquid flowing into the upper portion of the draft tube during part load turbine operation, so as to mitigate breakdown of the vortex flow path.

FIELD OF THE INVENTION

The present invention relates to a hydraulic turbine having one or morecontrol jets of liquid injected with high velocity axially downstream ofthe turbine runner and into an upper portion of the turbine draft tube,during part load operation of the turbine, to control the swirling flowand mitigate both helical vortex breakdown and its associated pressurefluctuations.

BACKGROUND OF THE INVENTION

Until recently, hydraulic turbines have been operated close to peakefficiency. In the neighborhood of this optimum operating point, dynamicforces on the turbine components are generally low, with the exceptionof transient conditions such as load rejection and surge.

The variable demand on the energy market, as well as the limited energystorage capabilities, requires a great flexibility in operatinghydraulic turbines. As a result, hydraulic turbines tend to be operatedover an extended range far from the best efficiency point. Inparticular, Francis turbines, which have a fixed-pitch runner, have ahigh level of residual swirl at the draft tube inlet as a result of themismatch between the swirl generated by the wicket gates (guide vanes)and the angular momentum extracted by the turbine runner when operatingat part load conditions. In the turbine draft tube the flow exiting therunner is decelerated, thereby converting the excess of the kineticenergy into static pressure. The decelerated swirling flow often resultsin breakdown of the normal vortex associated with flow of liquid in thedraft tube which gives rise to the development of a centralquasi-stagnation region in the draft tube. The vortex breakdown is nowrecognized as a primary cause of severe pressure fluctuations orpressure pulsations experienced in the draft tube of a hydraulic turbineoperating at part load. The pressure pulsations are believed to becaused by the transformation of an axis-symmetrically swirling vortexflow into one or more precessing helical vortices as the operatingcondition shifts towards part load. The precessing motion of the helicalvortex results in a fluctuating pressure on any stationary point of thedraft tube. In addition, a limited quantity of air or water vapor in theliquid flow provides a degree of elasticity, termed cavitationcompliance, and this elasticity can lead to a form of resonance in thedraft tube excited by the precessing inhomogeneous pressure fieldassociated with the core of the spiral vortex flow.

Many different solutions have been proposed with respect to the problemof draft tube instability including altering blade design, theintroduction of vanes in the draft tube, and the injection of air into arecirculation region surrounded by the vortex rope. The air injectionproduces an essentially axis-symmetrical stable flow, or a hollow aircore surrounded by the swirling water flow. The air injection changesthe breakdown of the vortex form from a spiral to a bubble. Theinjection of relatively small amounts of air have small effects on theefficiency of the turbine operation while considerably reducing the partload pressure swings. However, the vortex rope and the excitation of therope continues to exist.

Accordingly, there is a need to develop hydraulic turbines for presentday hydroelectric facilities that operate efficiently not only undernormal load conditions but also at low or partial load conditions,without being subjected to the severe pressure fluctuations originatingin the draft tube as a result of helical vortex breakdown in thedecelerated swirling flow downstream of the runner.

SUMMARY OF THE INVENTION

The present invention relates to controlling swirling flow downstream ofa hydraulic turbine runner by the axial injection of high velocity jetor jets of liquid at the runner outlet or draft tube inlet. The controljet or jets of liquid act on the flow of liquid in the draft tube bymitigating breakdown of the vortex flow path of this liquid and therebydiminishing or eliminating draft tube pressure pulsations experiencedduring part load operation of the turbine.

By “high velocity” it is meant that the axially directed jet of liquidhas a velocity that is greater than the mean axial velocity of liquidflowing at the runner outlet so as to provide the benefit of the presentinvention. This high velocity of the liquid control jet or jets may beas much as about 2 to 4 times or more greater than the mean axialvelocity of the liquid flowing at the runner outlet. It should beunderstood that the high velocity of the liquid control jet or jets willvary depending on the location of injection of the liquid control jet orjets and the number of control jets utilized. It should be furtherunderstood that by making reference to a control jet or jets of highvelocity liquid being injected axially of the turbine it is meant thecontrol jet or jets, may be directed along the turbine axis, parallel tothe turbine axis, or converging on a focal point adjacent the upperportion of the draft tube lying on the turbine axis or a parallel axisadjacent to the turbine axis. Further the jet or jets may be locatedoffset from the turbine axis by, for example but not limited thereto, 10percent of the diameter of the turbine runner and still be considered tobe located relative to a central axis of the turbine runner.

The control jet or jets are preferably operated when the turbine isoperating at part load conditions. The control jet or jets are injectedfrom at least one nozzle head device positioned relative to a centralaxis for the turbine runner and adjacent to an upper portion of thedraft tube whereby the jet or jets of liquid are injected downstream ofthe runner. The at least one nozzle head device has at least one nozzlefrom which a corresponding high velocity liquid control jet is emittedinto the draft tube. In one embodiment, the turbine has a rotatablerunner mounted above the draft tube and the runner has a crown portionthat houses the at least one nozzle head device. In an alternativeembodiment, the at least one nozzle head device may be supported in anupper portion of the draft tube below and spaced from the crown of theturbine runner.

The at least one nozzle head device may comprise a single nozzle or aplurality of nozzles arranged in one or more circular arrays, or asingle annular nozzle.

In accordance with the present invention there is provided a hydraulicturbine comprising a passageway permitting liquid to pass through theturbine and a draft tube defining a portion of the passageway throughwhich liquid normally flows in a vortex flow path during optimal turbineoperating conditions. A rotatable runner is mounted upstream of thedraft tube and rotates about a central axis passing through the runnerand extending into the draft tube. At least one nozzle head device ispositioned relative to the central axis of the runner and adjacent to anupper portion of the draft tube. The at least one nozzle head device hasat least one nozzle from which a corresponding control jet of highvelocity liquid is injected axially downstream of the runner and intoliquid flowing into the upper portion of the draft tube during part loadturbine operation so as to mitigate breakdown of the vortex flow path.

In accordance with the present invention there is provided a method ofcontrolling part load operation of a hydraulic turbine during part loadconditions having a runner, a draft tube located downstream of therunner and a liquid passageway extending through the runner and thedraft tube. The method comprises the step of injecting one or morecontrol jets of high velocity liquid axially of the turbine, downstreamof the turbine runner and into at least an upper portion of the drafttube.

It is envisaged that the method may further include the step of locatingthe one or more control jets centrally of the runner prior to the stepof injecting. Further, the jet or jets may be located offset from theturbine axis.

Further during the step of injecting the one or more control jets, theone or more control jets may be injected in one direction selected fromthe group consisting of along an axis of the turbine, parallel to theturbine axis, converging on a focal point adjacent the upper portion ofthe draft tube lying on the turbine axis and a parallel axis adjacent tothe turbine axis.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature and objects of the presentinvention reference may be had to the accompanying diagrammatic drawingsin which:

FIG. 1A is an elevation view, partially in cross-section, of a Francisturbine showing a nozzle head device for emitting a control jetpositioned in the crown above the draft tube;

FIG. 1B illustrates an alternative embodiment for the nozzle head devicefor emitting a control jet where the nozzle head device is in the runnercrown exclusively;

FIG. 2 illustrates an alternative embodiment for the nozzle head devicefor emitting a control jet where the nozzle head device is positioned inthe draft tube spaced below the crown;

FIGS. 3A and 3B show comparative velocity contours for water flowingthrough the draft tube;

FIGS. 4A and 4B show comparative pressure contours for water flowingthrough the draft tube;

FIGS. 5A through 5D show alternative embodiments for the arrangement ofnozzles in the nozzle head device.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a hydraulic turbine having one or moreliquid control jets axially directed downstream of a turbine runner andinto an upper portion of a draft tube. The present invention is intendedfor use in fixed-pitch hydraulic turbines and preferably findsapplication in propeller and Francis type turbines.

Referring to FIG. 1A there is shown an exemplary hydraulic turbineinstallation 10 suitable for use in the generation of hydro-electricity.The turbine installation 10 comprises a Francis turbine 12 having acrown 14, runner blades 16, and a band 18. The Francis turbine runner 12is adapted to rotate within a stationary casing 42. Below the Francisturbine runner 12 is located a draft tube 22. It should be understoodthat while a Francis turbine runner is shown the runner could also be apropeller type runner. Only a portion of the draft tube 22 is shown. Theupper portion 24 of the draft tube 22 is shown to have a verticallyextending central axis 26. The axis 26 is also the central axis for therunner 12 and the axis of the hydraulic turbine 10.

The upper portion of the crown 14 is connected to shaft 28. The shaft 28has a coupling flange 30 which is connected by bolts 32 to a couplingflange 34 of a generator shaft 36. Rotation of the Francis runner 12causes the shaft 28 to rotate and hence, the generator shaft 36 torotate. The generator shaft 36 is connected to a generator (not shown)which generates electricity in response to the rotating action of theFrancis turbine 12.

The Francis turbine 12 rotates as a result of water moving along waterpassageway 40, from the spiral casing 42 past stay vanes 44, past wickedgate 46, the runner blades 16 and into the draft tube 22.

In accordance with the embodiment shown in FIG. 1A, a control jet 50 isinjected into the water flowing in the top portion 24 of the draft tube22. A nozzle head device 52 comprise an end portion of conduit 58 thatpasses through the centre of the crown 14. The head device 52 has anozzle 54 from which the control jet 50 is injected into the water orliquid flowing through the draft tube 22. The crown 14 has a crown tipportion 56 that houses the nozzle head device 52.

Water under pressure is supplied to the crown tip portion 56 and thenozzle head device 52 by the conduit 58 passing through shaft 28. Theconduit 58 is connected to radially inwardly extending conduits 60. Theshaft 28 has an outside wall 62 with one or more liquid ports 64contained therein. The radially inwardly directed conduits 60 areconnected with the nozzle 54, via conduit 58 and nozzle head device 52and transport high pressure water from the liquid ports 64 to the nozzle54. In the illustrated embodiment the inlet ports 64 are located betweenthe coupling flanges 30 and 34 which is also referred to herein as beinglocated on the outside wall of the shaft 28 as the coupling 30 formspart of the shaft 28. While the shafts 28 and 36 are illustrated asbeing solid, in practice, they are typically hollow.

A stationary liquid collection chamber 66 is mounted in surroundingsealing relation with the hollow shaft 28, or in the illustratedembodiment the coupling flanges 30, 34. Stationary liquid collectionchamber 66 directs pressurized water into the ports 64. The stationaryliquid or water collection chamber 66 is connected to a liquid or waterbypass supply conduit 68 at an end portion 70 thereof. The liquid bypasssupply conduit 68 has an opposite end portion 72 coupled in sealingrelation with the spiral casing 42 and communicates liquid from thespiral casing 42 to the liquid collection chamber 66. A regulating valve74 is located in the pressurized liquid supply conduit 68. The valve 74controls the flow rate of the liquid supplied to the liquid chamber andis adapted to switch the control jet from an off condition to an oncondition and to vary the flow rate of the control jet depending on thepart load operating conditions of the turbine installation 10. In theembodiment shown, the regulating valve 74 is located upstream of thenozzle 54 so as to control the flow rate of the water and hence theresultant velocity of the “high velocity” control jet 50 emitted fromnozzle 54. It should be understood that the regulating valve 74 is ableto switch the supply of water to liquid collection chamber 66 off whenthe turbine is operating at optimal load conditions. As a result nowater is emitted from nozzle 54. In the embodiment of FIG. 1A, thecontrol jet 50 is emitted from the nozzle 54 of nozzle head device 52which is positioned centrally of, and adjacent to, upper portion 24 ofdraft tube 22 within the crown 14. Producing the water jet at the crowntip takes advantage of the hollow turbine shaft, and benefits from ahigh-pressure water supply from upstream in the turbine spiral casing.

It should be understood that FIG. 1A illustrates an exemplary jetgeneration and control system comprising head device 52, conduits 58,60, water collection chamber 66, conduit 68, and valve 74 and thatalternative embodiments for supplying water under pressure to the headdevice 52 will be readily understood to a person skilled in the art.

In FIG. 1B there is shown a cross-section through a turbine 10comprising a spiral casing 111, stay vanes 113, guide vanes 112, Francisrunner 109, and draft tube 114. A passageway 115 extends through theturbine 110. The runner crown 120 comprises a central cavity 122connected with the high pressure side 124 of the runner 109 by opentubes 126. A nozzle head device 128 has a needle 132 and a nozzle 130from which the high velocity liquid control jet 50 is emitted into theupper portion 140 of the draft tube 114 from the runner crown 120. Anadjustment mechanism 134 is driven by an active control device 136 foradjusting the position of the nozzle head device 128 so as to controlthe speed of the high velocity liquid control jet 50.

One or more pressure sensors 142 are mounted in the draft tube 114adjacent a sidewall thereof near the upper portion 140 of the draft tube114. The pressure sensors 142 measure the water pressure in the upperportion of the draft tube 114 and relay these measurements to the activecontrol device 136 which in turn controls movement of the nozzle headdevice 128 in order to adjust the velocity of the high velocity liquidcontrol jet 50. When in an open position for the nozzle head device 128,water from the high pressure side 124 of the runner 109 is suppliedthrough tubes 126 the cavity or water chamber 122 to the nozzle 130. Asa result a liquid control jet 50 will be emitted from the crown 120 ofthe runner into upper portion 140 of the draft tube 114 also known asthe draft tube inlet. The velocity or flow rate of the liquid controljet 50 is controlled by the position of the nozzle head device 128depending on the pressure measured by the pressure sensors 142. Thenozzle head device 128 is closed when the turbine is not operating atpartial load operating conditions. This system of jet generation has theadvantages that the jet generation system concerns a single component ofthe turbine, namely the runner. The system of generation does not implyany water tightness problems, and can be implemented in situ forexisting runners and all the system components, except for the runnercavities and tubes and can be considered to be a turnkey jet generationtype of kit.

Referring to FIG. 2 there is shown an alternative embodiment wherein thenozzle head device 80 is spaced from the crown 14 of the turbine 10. Thenozzle head device 80 comprises a nozzle 82 from which the liquid jet 50is injected vertically axially along the axis 26 of the draft tube 22adjacent the upper portion 24 of the draft tube 22. The nozzle headdevice 80 further comprises a plurality of supporting and liquidsupplying conduits 84 interconnecting the head 82 with manifold 86located on the outside wall of the draft tube 22. The manifold 86 isconnected via bypass supply piping 88 to the scroll 40. A regulatingvalve 90 is located to control the pressure of the liquid or watersupplied to nozzle 82 and hence the “high velocity” of the liquidcontrol jet 50. In this embodiment, water under pressure is suppliedfrom the scroll 40 through the wall of the draft tube 22 and not throughthe crown 14 of the Francis turbine 12.

Referring to FIGS. 3A, 3B, 4A and 4B there are shown computersimulations of velocity and pressure contours of water flowing in thedraft tube 22 that occur at part load conditions. FIGS. 3A and 3B showvelocity contours of water flowing within the draft tube at part loadconditions. In FIG. 3A, no control jet has been injected into the drafttube. In FIG. 3B, a liquid control jet has been injected into the drafttube. In FIG. 3A, where no jet is employed, there is shown a singlehelical draft tube vortex rope 94. In FIG. 3B where the control jet isin operation, the central low pressure region indicated by theiso-surface has been greatly reduced and its shape has changed from ahelical shape to a slightly off-centre extended cone 96. In FIG. 4A, thejet is not employed and the pressure contour shows strongcircumferential variations in the vortex flow of the water in the drafttube at 98, which associated with the precession motion result in severepressure fluctuations. In FIG. 4B, where the control jet is employedthere appears to be no breakdown in the low pressure area associatedwith the vortex at area 100.

By injecting a control jet of high velocity liquid axially into thedraft tube, the precession frequency is altered in the draft tube and,eventually, by eliminating the quasi-stagnant central region, thecontrol jet prevents or reduces development of vortex ropes in the drafttube liquid flow. As a result, the control jet addresses directly thevortex rope occurrence development thus mitigating the main source ofpressure fluctuations, or at least it alters the precession frequencyand reduces significantly the pressure fluctuations amplitude. Injectinga control jet of liquid is different from the air admission at the tipof the crown since the control jet of liquid is aimed at controlling oreliminating the vortex breakdown. Further when the control jet is notneeded during turbine operation, the control jet can be switched off.

By avoiding the helical vortex breakdown the overall performance of thedraft tube at part load is significantly improved by reducing thehydraulic losses due to severe flow non-uniformities and unsteadiness.

The control jet provides an active control of the swirling flowdownstream of the runner. The control jet uses a fraction of the overallturbine discharge. The jet discharge bypasses the turbine bladed regionand produces no power at the turbine shaft. However, the reduction inefficiency as a result of the jet discharge bypassing the turbine bladedregion is lower than expected. This is because of reduction of hydrauliclosses due to the precessing vortex rope compensating for hydraulicenergy spent on the jet. In addition, the control jet has the benefit ofdiminishing the severe pressure pulsations and the draft tubeinstability at partial discharge.

Referring to FIGS. 5A through 5D there is shown embodiments of theheader portion 82 for a single nozzle head device 80 from FIG. 2. Itshould be understood that multiple nozzle head devices could be employedor multiple head portions 82 for each nozzle head device could beemployed for alternative embodiments. However, in FIG. 5A, a singlenozzle 102 for emitting the control jet 50 is located to direct the jet50 along axis 26 of the draft tube. Alternatively, this jet 50 could bedirected along an axis parallel to and offset from the axis 26. In FIG.5B, the plurality of nozzles 102 are arranged in a circular array aboutthe central axis 26. This will result in a plurality of jets beingemitted from the nozzles 102. Alternatively the jets could be arrangedin a circular array an axis parallel to and offset from the axis 26. Thenozzles 102 can be arranged to emit the jets either parallel to the axis26 or the jets may converge towards each other with a focal point lyingon the axis 26, or the jets may be directed to focus on an axis parallelto the central axis 26. In FIG. 5C, two circular arrays of nozzles 102are arranged concentrically about the central axis 26. In FIG. 5D, asingle nozzle in the form of annular ring 102 is arranged around theaxis 26. In alternative embodiments to FIGS. 5A to 5D, the location ofthe nozzles can be chosen to direct the control jet or jets to be offsetfrom the control axis 26 by as much as 10% of the diameter of the drafttube 22.

The utilization of the control liquid jet or jets of the presentinvention: a) successfully addresses directly the main cause of the flowinstability, rather than the effects; b) does not require geometricalmodifications of the runner outer shape; c) is continuously adjustableaccording to the operating conditions, and can be switched-off when itis not needed; and, d) although a fraction of the discharge may bypassthe bladed region, the overall turbine efficiency suffers marginally,and may be improved, due to improvement in both runner and draft tubeefficiencies when the control jet is on at part load operatingconditions.

While the invention has been described in connection with what ispresently considered to be the most practical embodiments of thehydrodynamic approach of controlling the swirling flow and mitigatingthe helical vortex breakdown together with the associated severepressure fluctuations by using axial high velocity liquid control jet orjets, it is to be understood that the invention is not to be limitedthereto, but on the contrary, is intended to cover various modificationsand equivalent arrangements as would be understood by a person skilledin the art of hydraulic turbines.

1. A hydraulic turbine comprising: a passageway permitting liquid topass through the turbine; a draft tube defining a portion of thepassageway through which liquid normally flows in a vortex flow pathduring optimal turbine operating conditions; a rotatable runner mountedupstream of the draft tube and rotating about a central axis passingthrough the runner and extending into the draft tube; at least onenozzle head device positioned relative to the central axis of the runnerand adjacent to an upper portion of the draft tube, the at least onenozzle head device has at least one nozzle from which a correspondingcontrol jet of high velocity liquid is injected axially downstream ofthe runner and into liquid flowing into the upper portion of the drafttube during part load turbine operation, so as to mitigate breakdown ofthe vortex flow path.
 2. The hydraulic turbine of claim 1 wherein the atleast one nozzle locates the corresponding control jet of high velocityliquid in one position of along a central axis of the turbine and offsetfrom the turbine central axis.
 3. The hydraulic turbine of claim 1wherein the runner has a crown portion and the crown portion houses theat least one nozzle head device.
 4. The hydraulic turbine of claim 3wherein the runner comprises a jet generation and control system locatedin one of the crown portion and a shaft portion for the runner.
 5. Thehydraulic turbine of claim 3 wherein the at least one control jet ofhigh velocity liquid is ejected from the nozzle axially into a centralportion of the vortex flow path of the liquid flowing in the draft tube.6. The hydraulic turbine of claim 1 wherein the at least one nozzle headdevice further comprises at least one valve located upstream of the atleast one nozzle for controlling the pressure of the liquid of thecorresponding control jet.
 7. The hydraulic turbine of claim 1 whereinthe at least one valve is able to switch the corresponding control jetbetween on and off states.
 8. The hydraulic turbine of claim 1 the atleast one nozzle head device is supported in liquid flow communicationwith conduit extending from the draft tube wall into an upper portion ofthe draft tube.
 9. The hydraulic turbine of claim 1 wherein furthercomprising one or more pressure sensors in the draft for measuring theliquid pressure in the upper portion of the draft tube and relayingthese measurements to an active control device which in turn controlsmovement of the nozzle head device in order to adjust the velocity ofcontrol jet of high velocity liquid.
 10. A method of controlling partload operation of a hydraulic turbine during part load conditions havinga runner, a draft tube located downstream of the runner and a liquidpassageway extending through the runner and the draft tube, the methodcomprising the step of injecting one or more control jets of highvelocity liquid axially of the turbine, downstream of the turbine runnerand into at least an upper portion of the draft tube.
 11. The method ofclaim 10 comprising the step of locating the one or more control jetscentrally of the runner prior to the step of injecting.
 12. The methodof claim 11 wherein during the step of injecting the one or more controljets of high velocity liquid, the one or more control jets are injectedin one direction selected from the group consisting of along an axis ofthe turbine, parallel to the turbine axis, converging on a focal pointadjacent the upper portion of the draft tube lying on the turbine axis,and converging on a focal point adjacent the upper portion of the drafttube lying on a parallel axis adjacent to the turbine axis.
 13. Themethod of claim 11 wherein the step of locating comprises locating thecontrol jet or jets of high velocity liquid offset from a turbine axis.